Carbon dioxide (CO2) is the primary anthropogenic (man-made) greenhouse gas in terms of concentration and effect. It is responsible for about one-third of the global warming that derives from human activities.  Concentrations of CO2 in the atmosphere depend on the global carbon cycle that accounts for the fluxes of carbon among various storage pools. The vast majority of carbon on Earth is part of limestone and other sedimentary rocks. The decomposition of rocks, called weathering, as well as the high temperatures used during cement production, release CO2 from limestone and enters the atmosphere or dissolves in bodies of water.
Photosynthetic organisms absorb CO2 from the atmosphere or bodies of water and then use solar radiation to convert this low-energy carbon into the high-energy carbon in organic compounds. This process reverses when organisms breathe in and out (respire) or when they die and decompose. Thus, the CO2 released once more enters the atmosphere or dissolves in bodies of water. Fossil fuels come from organic carbon compounds formed during the decomposition of prehistoric organisms, and in turn, the burning of fossil fuels rapidly oxidizes these compounds and releases CO2.
Carbon exchanges among the atmosphere, soils, vegetation, and changes in land use (e.g., clearing of rainforest for agriculture) are balanced, meaning there is no net change in the amount of CO2 present. Exchanges among the atmosphere, surface ocean, marine biota, and dissolved organic carbon are similarly balanced. Carbon extractions from sedimentary rocks and from coal, oil, and gas deposits and the subsequent carbon emissions from fossil-fuel burning and cement production are out of balance, meaning there is a net release of CO2. Calculations of purchases of petroleum, natural gas, and coal indicate that global CO2 emissions from the burning of fossil fuels increased by 58% from 1980 to 2006.
Predictions of carbon emissions must take into account the contributions of various economic sectors and their potential expansion. Various scenarios about future demographic, social, economic, technological and environmental conditions result in a range of predicted global carbon emissions, which are frequently used in the Intergovernmental Panel on Climate Change (IPCC) reports. Scenarios that assume rapid economic growth and reliance on fossil fuels (A1FI) or regional autonomy (A2) predict a near tripling of the rate of carbon emissions during the twenty-first century. In contrast, scenarios that assume the adoption of alternative energy sources (A1T or B1) predict a rise in emissions until the middle of the century and then a decline. For the intermediate scenarios (A1B and B2), carbon emissions are predicted to approximately double and then level off.
The carbon released by human activities ends up in several places. Most of this carbon remains as CO2 in the atmosphere and atmospheric CO2 concentrations may increase by 50% to 300% to somewhere between 550 and 970 parts per million (ppm) during the twenty-first century. A portion of this carbon dissolves in the oceans, and vegetation assimilates some of this carbon through photosynthesis, and organic carbon compounds have accumulated on land and ocean floors. Global warming, however, will accelerate respiration and thereby will release more CO2 from this organic carbon. Some Global Climate Models (GCMs), such as the Hadley Climate Model, predict that by the latter half of the twenty-first century, respiration will exceed photosynthesis, and therefore land masses will become sources rather than sinks for carbon.
 Hansen, J. E. and M. Sato (2001) Trends of measured climate forcing agents. Proceedings of the National Academy of Sciences of the United States of America 98:14778-14783.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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